The present invention relates to a heat resistant sheet comprising wholly-aromatic polyamide fibers alone and the object of the present invention is to provide a novel heat resistant sheet having good impregnability with various varnishes, excellent dielectric strength and a high physical strength.
Conventionally, synthetic heat resistant sheets having paper structure have been used in electrical apparatuses, especially electric motor, generator, etc. having a dry insulating constitution and in this case the sheets have been subjected to calendering treatment for making the paper surface dense to prevent decrease in dielectric strength which is caused by pinholes. As the result, the varnish impregnability is decreased in an insulating system in which use of an insulating varnish is essential and dielectric strength is reduced and life of the apparatuses is also decreased. Therefore, various proposals have been made for improving impregnability of sheet with insulating varnishes by examination of paper making conditions, but sufficient improvement has not yet been attained.